1. Field of the Invention
This invention relates to a semiconductor integrated circuit device and more particularly to a semiconductor integrated circuit device having an electrically rewritable nonvolatile semiconductor memory.
2. Description of the Related Art
The voltage used to control the gate of a transistor which transfers charges in a charge transfer sense amplifier used in a NAND flash memory or the like is called the bit line clamp voltage.
The bit line clamp voltage is set to the potential used to precharge the bit lines at the read time and set to the potential used to transfer charges from the bit line to the sense amplifier in the sensing operation after the end of precharging.
The bit line clamp voltage is generated by a bit line clamp voltage generating circuit. In the generating circuit, an operational amplifier (hereinafter referred to as an op-amp) which includes intrinsic transistors. The intrinsic transistor is a low-threshold voltage transistor having a threshold voltage of zero or approximately zero. The reason why the op-amp including intrinsic transistors is used in the bit line clamp voltage generating circuit is that it is necessary to generate low potential equivalent to potential Vsen (approximately 0.35 V) used to determine “1” or “0” by means of the sense amplifier.
However, a variation in the threshold voltage of the intrinsic transistor caused in the manufacturing process is large. Therefore, the operable voltage range of the op-amp will be limited when taking the temperature dependency of the transistor into consideration. As the whole portion of the circuit, the operable voltage range of the op-amp is limited and it becomes difficult to attain a sufficiently wide operable voltage range.
For example, the threshold voltage of the transistor becomes higher at low temperatures. Further, if a variation in the threshold voltage deflects to a higher level, the threshold voltage may exceed the input voltage Vclampref of the op-amp, for example, 0.35 V in some cases. If such a state occurs, the op-amp is not operated.
On the other hand, if a variation in the threshold voltage deflects to a lower level, the transistor of the op-amp cannot satisfy the relation of Vgs−Vth<Vds and does not perform the saturation region operation. Like the above case, the op-amp is not operated.